This invention relates to Time Division Multiplexing/Time Division Multiple Access (TDM/TDMA) communications.
In systems employing TDM/TDMA for the transmission of signals, TDM is typically used to transmit signals downstream from a network end to a plurality of end-user terminals at a home or business over a single channel. At the home or business end, a receiving terminal receives all downstream transmissions that are directed both to it and the other end-user terminals. However, in each received frame of data bytes, only those bytes that are properly intended for a particular receiving terminal are delivered to that terminal for processing. Typically, this can be done by assigning different time-slots in each frame to specific receiving terminals. Each receiving terminal thus only xe2x80x9clooksxe2x80x9d in its assigned time-slot for the bytes directed to it. Alternatively, the downstream signal, if originating, for example, from a broadband asynchronous transfer mode (ATM) network, may consist of a sequence of ATM cells which each include header information indicating an address of the destination(s) to which the cell is directed. A receiver terminal then only xe2x80x9cpicks outxe2x80x9d the ATM cells that are addressed to it or are broadcast to many, and discards the other ATM cells addressed elsewhere.
In the upstream direction, TDMA transmission is used for transmitting the outputs of multiple end-user terminals back to the network end. One way this is implemented is to allow an end-user terminal to transmit back to the network end only during a specific time-slot each frame. At the network end, therefore, the bytes received from the multiple end-user transmitting terminals are demultiplexed into separate plural data streams in accordance with the time-slots each frame during which they are received.
Whereas the one-to-many aspect of TDM downstream transmission on a single channel is implemented in a relatively straight-forward manner at both the network end and the end-user terminal end, upstream transmission from a plurality of end user terminals to a single network end presents several technical difficulties with respect to the management of the available upstream bandwidth. This is particularly true in digital broadband access networks that employ optical fiber-to-the-home (FTTH), utilizing a power splitting passive optical network (PSPON) topology. As presently configured, each PSPON fiber can support up to 32 homes or businesses. In such a system, bidirectional communications over a single fiber is achieved using coarse wavelength division multiplexing (CWDM), in which one wavelength, 1550 nm, is used for downstream communications to all home/business end user terminals that are connected to that fiber. Another wavelength, 1310 nm, is then used for transmission to the network end of upstream data from all those connected homes/business terminals. That data, in both directions, can include video, data (e.g., Internet-type data), and digitized voice. In such systems, the fiber is terminated at the home/business by an Optical Network Unit (ONU), and at the network end by an Optical Line Card (OLC).
In such systems, ATM in a frame structure is employed for downstream transmission while ATM transmitted in bursts is used for upstream transmission. Prior art PSPON based systems use a framed structure, in the downstream direction, that consists of 2968 bytes at a bit rate of 155.52 mbits/sec. These 2968 bytes in each downstream frame, represent 56 ATM cells/frame, each cell consisting of a 5-byte header and a payload of 48 bytes. Addressing information is included within that 5-byte header, which enables each of the up to 32 end user ONUs to select for reception by its connected terminals only those ATM cells that are broadcast or specifically addressed to it. In the upstream direction, the ONUs transmitting to the OLC consecutively transmit bursts, each burst containing a single ATM cell. Assuming an additional 3-byte burst header, each burst is thus 56 bytes long. Each 2968 byte frame, therefore, contains 53 bursts, each being 56 bytes. If each ONU transmits one burst per frame, approximately 2.777 mbits/sec of user ATM upstream bandwidth is available to each end user terminal. Disadvantageously, a finer bandwidth granularity (e.g., less than 2.777 mbits/sec) per end user terminal requires assigning p bursts every m frames, requiring a complicated upstream bandwidth management procedure. Further, if an end user terminal requires a higher upstream bandwidth (e.g., higher than 2.777 mbits/sec), the ONU must manage multiple bursts per frame from such a terminal. Digital voice communications in prior art system has further inefficiencies. Specifically, since the bandwidth requirement for a digital voice channel is only 64 kbits/sec (equivalent to one byte per frame), 47 of the 48 payload bytes in each ATM cell containing digitized voice in each upstream burst remain unused (assuming one burst every 125 xcexcsec). If, alternatively, 48 voice samples are accumulated over a 6 msec period before being transmitted, echo cancellation will likely need to be implemented due to the delay imposed on the transmitted voice samples. Furthermore, the 8000 samples/sec associated with digital voice circuits cannot be simply generated from the non-8000 frames/sec frame rate.
A need therefore exists to better control the bandwidth allocation to end user terminals of all types, and especially as applied to digital transmission of voice signals.
In accordance with the present invention, efficient bandwidth allocation is achieved by using variable length bursts for upstream transmission. Rather than setting the length of each upstream burst at a fixed length, the length of each burst is determined in accordance with the actual bandwidth requirements of the transmitting end user terminal. In particular, and depending upon the overall bandwidth requirements of all the end user terminals transmitting over the upstream channel, the number of payload bytes per burst can vary between zero and the total number of payload bytes allocated per frame. The latter would occur if only one end user terminal is connected to the channel for upstream communication. In the more likely scenario of multiple end user terminals communicating over the channel, the total number of bytes per upstream frame are divided among all the end user transmitting terminals in accordance with their current bandwidth requirements and the overall bandwidth capacity of the channel. Each end user transmitting terminal then transmits one and only one burst each frame. That burst contains all the digital information that the end user is transmitting upstream to the network end including, for example, video, data, and digital voice. Advantageously, a high degree of granularity in allocating bandwidth can be achieved since the burst length can be adjusted in one byte increments.
For the specific embodiment of the PSPON topology in which frames are conveniently transmitted at 8000 frames per second and in which the upstream frame comprises 2430 bytes, bandwidth can be assigned in one bytexc3x978000/sec increments or equivalently, 64 kbit/sec increments. Advantageously, included within the payload of each upstream burst transmitted by the ONU that is connected to one or more end user terminals then being used, is one byte per each active digital voice channel required by that end user. Thus, if there is no current active telephonic conversation, no bytes are used, whereas if one voice channel is active, a single byte in the upstream burst is allocated to and used for digital voice transmission. Additional voice circuits associated with that same ONU are transmitted in additional bytes in the upstream burst. The bandwidth allocated for each such digital voice channel is thus an efficient 64 kbit/sec for the specific example of a frame arrangement noted above.
In order to manage the allocation of upstream bandwidth among the plural end user terminals transmitting upstream information, the terminating terminal associated with each end user, such as the ONU, is assigned the timing and length of its upstream TDMA slot. This is effected through the broadcast of downstream cells containing slot assignment or modification messages to the terminating terminal. The slot assignment messages include information used for assigning a slot of a specified length to a particular end user terminating terminal. Such information thus includes where (i.e., from which byte position) within each upstream frame that the terminating terminal is to transmit, and how many bytes the payload of each burst from that terminal is to be. An assignment message is used to assign an upstream slot to a newly installed end user terminating terminal and to reassign (i.e., confirm) an existing assignment as a fault recovery mechanism. A modification message is used to change the length of an existing slot assignment and/or the number of digital voice channels associated with the terminating terminal to which the message is directed. Such a message, by necessity, is also used to move the location of all assigned slots located after that modified slot in the frame.